The discovery of mutated mismatch repair genes from families with hereditary nonpolyposis colon cancer (HNPCC) and in some sporadic colon cancers has now linked defects in DNA repair with a common disease. Little is known about how these genes may prevent tumorigenesis. Our hypothesis is that the mismatch repair system serves as the "afferent limb" of a DNA-monitoring system that not only recognizes and repairs DNA nucleotide pair mismatches, but triggers a G2 cell cycle arrest and cell death in response to alkylation DNA damage. We have developed a model to test this hypothesis and to study the regulation of proteins responsible for cell entry into mitosis in response to DNA damage. We will examine two of the mismatch repair genes, hMLH1 and hMSH2, to determine if there are differences in specificity of cell cycle regulation between the two genes. We will also determine whether the presence of two copies of normal mismatch repair genes affects commitment of the cell to growth arrest or cell death in response to DNA damage, and what role genes that influence immortalization and regulate cell death play in mismatch repair-competent cells. An understanding of the molecular events involved in DNA damage repair, cell cycle control, and programmed cell death will enhance our understanding of the development of the cancer cell, and may promote development of strategies by which to prevent the initiation and progression of cancer. This application is designed to pursue advances in our understanding of the DNA mismatch repair genes as well as provide a foundation for the P.I's growing scientific career. The P.I. is dedicated to a career in academic medicine, and has achieved recognition of his research by receiving the Commonwealth Fund Medical Research Fellowship, and Distinction in Biomedical Research and Department of Medicine Research Awards while performing studies in medical school. Since the completion of his clinical training, the P.I. has engaged in basic science research gin the laboratory of C. Richard Boland, M.D., a leader in the field of gastrointestinal oncology. Dr. Boland's research focuses on mechanisms of tumorigenesis, including the biology and genetics of colon cancer, and the progression of tumor markers in malignant transformation. Ongoing studies in Dr. Boland's laboratory complement the proposed studies, creating a stimulating setting that will enhance the P.I.'s scientific education. Outstanding scientific resources at the University of Michigan, including those offered by the Comprehensive Cancer Center, will allow the P.I. to develop collaborations, participate in conferences, and to explore new technology.